An output video signal from various video signal sources, such as a TV receiver or a microprocessor or a video disk, presents variations on its peak to peak value. Therefore, the peripheral device using this video signal, for example a tape recorder for the TV receiver, has to comprise an AGC device for reducing this amplitude variation.
Conventional AGC devices are analog loop systems which compare a peak or quasi-peak value of the signal to a reference value. Such devices are chosen so that the gain modification time constant is sufficiently long, for example a plurality of lines of the video signal so that the video signal is not impaired during the gain modification. For this purpose, large capacitors are generally used and are not integrable. It is therefore necessary to add a capacitor externally to the integrated circuit and to provide for an additional connection pin in this purpose. However, when the image content is rapidly changing, distortion and overoscillation may occur. Therefore, very often, for improving the image quality, the time constant has to be shorter in case of gain reduction than in case of gain increase, and the circuit external to the integrated circuit has to be more complex.
A need therefore exists for an AGC circuit for video signals which avoids signal distortion and circuit overoscillation when the image content of the signal rapidly changes.
A need further exists for an AGC circuit for video signals which does not require an external capacitor.
Accordingly, an object of the invention is to provide an AGC circuit for video signals which has a time constant responsive to circuit gain variations.
Another object of the invention is to eliminate the need for an external capacitor.